Nearshore Ooid Formation in a Modern Temperate Region Marl Lake

Low magnesian calcite ooids growing on shallow lake-margin bench platforms of Higgins Lake in central Michigan represent the first reported occurrence of ooid formation in a modern low-salinity lacustrine system. Ooid cortex accretion about palimpsest medium sand nuclei is restricted to shallow water depths and to a narrow belt which lies parallel to the lake margin. Landward, terrigenous grains derived through erosion of glacial outwash predominate on the platform surface. Lakeward, lower energy conditions and infrequent agitation of terrigenous sands preclude ooid growth at these greater depths. Cortices of individual ooids are poorly laminated, exhibit highly irregular mucilagenous outer surfaces, consist of interlocking equant anhedral low-magnesian calcite crystals, and contain scattered unoriented endolithic cyanophyte filaments throughout. Fabric relationships between external mucilage, cortical carbonate, and cyanophytic filaments indicate that algae are largely passive during ooid accretion, while surficial organic mucilage may be critical to cortex growth.